Bayonetta is one sexy game. The main character, Bayonetta, is encased in a full-body suit made of human hair and structs around in high heels outfitted with pistols. See? That's hot. And is fertile ground for porn parody fan art.
However, Bayonetta designer Hideki Kamiya doesn't exactly seem too happy with that sort of thing, writing, "It seems there is an erotic doujinshi of Bayonetta...There are fans that find that sort of thing unpleasant, and I don't think it can't possibly show any sort of love for the game."
Unpleasant or not, doujinshi (think fan fiction) come with the territory and are often a good gauge of a game or character's popularity.